Control device for a refrigeration plant, and control method

ABSTRACT

A method of and apparatus for controlling a refrigeration plant having a cooler for cooling cooling air, the surface temperature of the cooler being measured by a single sensor and the cooling-air temperature being derived from the surface temperature of the cooler via a correction factor, the refrigeration plant then being controlled an the basis of the measured temperature value and the derived temperature value.

FIELD OF THE INVENTION

[0001] The invention relates to a control device for a refrigeration plant, especially for cold storage rooms, and to a method of controlling the temperature of the cold storage room.

BACKGROUND

[0002] In a refrigeration plant by means of which the temperature in a cold storage room is kept to a predefined value, it is known to provide a sensor to determine the temperature of the cold storage room, the said sensor generally being arranged in the air inlet upstream of the evaporator, and also to provide a sensor for determining the temperature of the cooler in the evaporator, this sensor generally being arranged on the surface of the cooler. During the design of a refrigeration plant, a temperature difference Δt₁ in Kelvin between the desired cold storage room temperature and evaporator temperature or else the temperature at which the refrigerating medium evaporates is defined. In the case of deep-frozen goods, for example, a temperature difference Δt₁ of 10 K is defined when designing the plant; in the refrigeration of foodstuffs such as vegetables, for example, a Δt₁ of 7° K. From this temperature difference, defined from the outset, the power calculation for the evaporator is carried out.

[0003] In such a refrigeration plant, the sensor for the cold storage room temperature controls, via a control unit, a fan for the passage of air through the cooler and the refrigerant circuit, for example the compressor, while the sensor on the cooler surface controls the final defrost temperature for a defrost heater and the like. At the end of defrosting, the compressor is then switched on first by the sensor for determining the cold storage room temperature and, after a predefined temperature has been reached, the fan of the evaporator is switched on by the sensor on the cooler surface. In the cooling cycles which then follow the fan of the evaporator and the compressor are again controlled by the sensor for the cold storage room temperature.

SUMMARY OF THE INVENTION

[0004] The invention is based on the advantage of constructing a temperature control system such that it keeps the cold storage room temperature to the predefined set point in a reliable way with a simple construction.

[0005] A single sensor is provided on the cooler surface, by means of which the cold storage room temperature is determined via a correction factor and the refrigeration plant is controlled on the basis of the determined cold storage room temperature, which results in a more simple construction as a result of omitting a second sensor for the cold storage room temperature. The fact that a single sensor is responsible for controlling the refrigeration plant reduces the investment costs, the outlay on installation and possible service costs. Fault sources are minimized by the fact that confusion between the sensors and the sensor connection lines is ruled out. Line-bound input coupling resulting from electromagnetic interference, such as can be coupled in sensor connecting lines and then have a detrimental effect on a control unit, is reduced by half by omitting one sensor. A control unit having only one sensor is considerably better in terms of its functional reliability as compared with the prior art, since all the measured variables are determined by a single sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The invention will be explained in more detail by way of example using the drawings, in which:

[0007]FIG. 1 shows a refrigeration plant for cooling in a cold storage room, in schematic form,

[0008]FIG. 2 is a graph illustrating control of the cold storage room temperature, and

[0009]FIG. 3 is a graph for determining the correction value.

DETAILED DESCRIPTION

[0010]FIG. 1 shows a cold storage room 100 and a refrigeration plant 12 having an evaporator 9 for the air cooling, in the housing of which there are arranged a fan 3 for the passage of air and also a cooler 8 comprising piping through which refrigerating medium flows and which has cooling fins 17, which preferably consist of aluminum. The numeral “6” designates an example of an electrical defrost heater, by means of which the cooler 8 is defrosted when iced up. The numeral “7” designates an expansion valve. Instead of electrical defrost heating, another form of defrost heating can also be provided.

[0011] In addition, the refrigeration plant 12 comprises a compressor 4, an air-cooled liquefier 10 and a refrigerating-medium collector 11. Arranged in the air-cooled liquefier 10 is a fan 13 and piping with cooling fins 16. Via a suction line 15, gaseous refrigerating medium flows from the cooler 8 in the evaporator 9 to the compressor 4, and liquid refrigerating medium flows through the liquefier 10 and the refrigerating-medium collector 11, through a line 14 and via solenoid valve 5, to the expansion valve 7.

[0012] Arranged on the cooler 8 of the evaporator 9 is a temperature sensor 2, by means of which the surface temperature t_(K) of the cooler 8 is determined. In a known refrigeration plant, a second sensor (not illustrated) is arranged in the room to be cooled or in the air inlet upstream of the evaporator, wherein the sensor measures the temperature of the cooling air.

[0013] In the configuration of a refrigeration plant according to the invention and shown in FIG. 1, only a single temperature sensor 2 is provided on the cooler surface, preferably between the cooling fins 17, by means of which sensor the surface temperature t_(K) of the cooler 8 is determined.

[0014] The numeral “1” designates a control unit TC which accepts the temperature value measured by the sensor 2 and, via an electronic control device, switches the fan 3, the compressor 4 and the defrost heater 6.

[0015] A program provided in the control unit 1 registers the surface temperature t_(K) of the cooler 8 and, in addition, determines the temperature of the cooling air or the cold storage room temperature via a correction value.

[0016] The correction value is determined as follows. For example, the refrigeration plant is to be designed for cooling vegetables to a cold storage room temperature of +2° C. In this case, the refrigeration plant is designed by the plant constructor in such a way that, for the set point of +2° C., an evaporation temperature to of −5° C. is ensured so that during practical operation, a Δt₁ of 7° K. is established. The Δt₁ is determined in a known way as the difference between the air inlet temperature t_(L1) and the evaporation temperature t₀. This is defined in the standards DIN 8955 and ENV 328 for determining cooler capacity. Values from experience show that a specific evaporation temperature t₀ of the refrigerating medium in the evaporator is to be allocated a specific value of the surface temperature t_(K) of the cooler in the evaporator.

[0017]FIG. 3 shows in schematic form the relationship between evaporation temperature to of the refrigerating medium and surface temperature t_(K) of the cooler. Such a relationship can be stored, for example, in the form of a table, in the electronic control unit 1. After the temperature difference Δt₁ has been defined from the outset, the predefinition of the desired temperature of + 2° C. and the resulting evaporation temperature of the refrigerating medium of −5° C. can be used to derive a surface temperature of the cooler t_(K) of −1° C. from FIG. 3.

[0018] This relationship results from the cooler construction and the design of the evaporator. From the surface temperature t_(K) of the cooler 8 in the evaporator 9, determined in this way, in this example a correction value of 3° K. is determined from the difference between the set point +2° C. and surface temperature t_(K) of −1° C. In other words, it is assumed that, according to the design of the refrigeration plant based on Δt₁, the cooling-air temperature t_(L) in this example lies above the surface temperature t_(K) of the cooler by the correction value of 3° K.

[0019] While for the purpose of cooling vegetables, for example, a set point of +2° C. and a correction value of 3° K. are used for example for the deep-frozen range, a set point of −20° C. is predefined, from which, via the predefined value of Δt₁=10°K., a correction value of 5° K. results. The value of Δt₁ has to be set on the control unit 1. The control unit 1 then determines the correction value from the set Δt₁.

[0020]FIG. 2 shows the temperature variation of the cold storage room temperature t_(L) and the surface temperature t_(K) at the cooler 8 over time, a set point for the cold storage room temperature t_(L) of +2° C. being assumed, such as is provided, for example, for vegetables as refrigerated goods. Starting from a switched-off state of the refrigeration plant, in which both the cooler surface temperature t_(K) and the cold storage room temperature t_(L) have a value lying above +2° C., the refrigeration unit 12 and the fan 3 in the evaporator 9 are switched on first by the control unit to bring the cold storage room temperature t_(L) to the set point. When the refrigeration unit 12 is running, the surface temperature on the cooler 8 is lowered as a result of circulation of the refrigerating medium in the refrigerant circuit. At the same time, the cold storage room temperature t_(L) is lowered by the running fan 3 in the evaporator 9. As soon as the sensor 2 detects a surface temperature t_(K) of −1° C., the control unit 1 determines that the set point t_(L) of +2° C. has been reached, by adding on the correction value of 3° K. This means that the control unit 1 switches off the compressor 4 and the fan 3. The fan 3 is switched on again when the sensor 2 indicates, via the correction value, a desired temperature t_(L) at the upper temperature value of +2.5° C. of a predefined tolerance range of ±0.5° K. about the desired temperature +2° C. in the control unit 1.

[0021] In FIG. 2, the tolerance range about the desired temperature +2° C. is reproduced by means of dash-dotted lines above and below the desired temperature. The compressor 4 is expediently switched off via the control unit 1 when the cooling air temperature t_(L), determined via the correction value, reaches the tolerance value lying below the desired temperature. The surface temperature t_(K) on the cooler 8 then rises again, the said temperature being determined via the sensor 2, the fan 3 of the evaporator 9 then being switched on again first, and then the compressor 4 being switched on again via the control unit 1 when the surface temperature t_(K) of the cooler 8 determined by the sensor 2 indicates the tolerance value of +0.5° K. lying above the desired temperature.

[0022] These cycles are repeated until, for example, as a result of icing of the cooler 8, the sensor 2 determines a surface temperature t_(K) from which icing of the cooler 8 can be derived in the control unit 1 by means of a comparison with predefined values or by means of a predefined program. At this point, the control unit 1 switches off the compressor 4 and the fan 3 and switches on the defrost heater 6 until the predefined final defrost temperature on the cooler 8 is again displayed via the temperature sensor 2. The defrost heater 6 is then switched off by the control unit 1, and the refrigeration unit 12 with the compressor 4 is switched on again. The cooling cycle illustrated in FIG. 2 begins again after the fan 3 of the evaporator 9 is likewise switched on again by the control unit 1 in accordance with a previously defined surface temperature of the cooler 8.

[0023] The temperature sensor 2 arranged in the cooler 8 constitutes a neutral measuring point which cannot be falsified by parameters such as is the case, for example, in a room temperature sensor whose measured value can be falsified, for example, by the fact that the room temperature sensor is covered by wrongly stacked refrigerated goods in the cold storage room. In this way, on account of the above-described control using only one sensor 2 via a correction factor starting from the previously determined Δt₁, the result is more reliable control than is the case in known refrigeration plants with two sensors of which the cooling-air sensor can be falsified by various parameters and false cooling-air temperatures can be determined. The temperature sensor 2 is arranged so as it is protected between the cooling fins, and cannot be damaged by putting refrigerated goods into storage and removing them.

[0024] Since the evaporator 9 represents a cold reservoir, and the surface temperature of the cooler 8 does not always reliably reproduce the cooling-air temperature in the room via the correction factor, for example, because the cold storage room temperature rises as a result of the transmission of heat from the refrigerated goods, without this having an immediate effect on the surface temperature t_(K) of the cooler 8, after a predefined time after the control unit 1 has established that the sensor 2 is indicating a set point in the tolerance range, the fan 3 is switched on so that room air is lead through the cooler 8 to check the actual room air temperature.

[0025] In this case, the compressor 4 is still switched off, since there is on the control unit 1 a signal from the sensor 2 which reproduces the cold storage room temperature within the tolerance range of the set point. Given warmer cooling-air temperature within the cold storage room in which, for example, good which have not yet been cooled have been subsequently store, the surface temperature t_(K) of the cooler 8 rises as a result of the warmer air brought up by the fan 3, until the surface temperature of the cooler has assumed the temperature of the room air. As a result, the sensor 2—without taking into account the correction factor—reports a current value of the room temperature which does not lie in the tolerance range of the desired temperature, for which reason the control until 1 switches on the refrigeration unit and the compressor 4 to take the surface temperature t_(K) of the cooler 8 back to a value which, with the correction factor, lies within the tolerance range of the desired temperature.

[0026] The room air temperature is measured indirectly by means of the sensor 2 via the surface temperature t_(K) of the cooler 8 and the correction factor. However, since the room air temperature can change more rapidly after reaching the desired temperature than can be determined by the only slowly following surface temperature on the cooler 8, the room air temperature must be repeatedly checked in this way or checked at specific time intervals by switching on the fan 3 again so that the control unit 1 can measure the actual room temperature via the sensor 2.

[0027] The fact that, after the desired temperature has been reached with the compressor 4 switched off, the fan 3 of the evaporator 9 is switched on first and the control unit 1 uses the sensor 2 to follow the temperature variation without correction factor, and the compressor 4 remains switched off until the control unit 1 uses the sensor 2 to determine a surface temperature t_(K) above the set point of +2° C., it being assumed that the surface temperature t_(K) of the cooler 8 has assumed the cold storage room temperature and only then is the compressor 4 switched on by the control unit 1, means that this method prevents the surface of the cooler 8 falling below the dew point at the time at which the compressor is switched on.

[0028] The advantageous features of the method outlined above are: considerably lower loss of mass of the refrigerated goods as result of reduced dehumidification of the cold storage room air. Considerable reduction in the energy consumption and, therefore, reduction in the operating costs as a result of utilizing the developed ice crystals as an energy store for cooling the room air. This results in an improved efficiency of the air cooler, shorter compressor running within the cooling cycles and, therefore, a longer useful life (service life) of the compressor. Defrost intervals are suspended by the pre-running of the fan, or the defrost cycles are reduced.

[0029] The above-described control of a refrigeration plant can be applied not only to cold storage rooms and deep-freeze rooms, but also to refrigerated and deep-freeze cabinets in which the fan corresponding to the fan 3 is permanently operating, and the cold air delivered by the fan corresponds to the room air, whose temperature is determined, via the correction factor, by means of the temperature sensor 2 fitted to the cooler 8. Refrigerated and deep-freeze cabinets of this type are used as island sales cabinets and chilled counters in commercial refrigeration and the like. This also applies in particular to room air-conditioning plant in the air-conditioning sector.

[0030] The above-described control of the room air temperature by means of a single sensor does not depend on the refrigeration media respectively used. For example, the evaporator 9 may also be an air cooler, which is operated not with direct expansion but with pumped refrigeration media in the form of liquid solutions, for example NH₃, or cold sols in two-circuit refrigeration plants, or Flo Ice or in CO₂ plants.

[0031] By means of the control unit 1, the solenoid valve 5 in composition refrigeration plants can also be driver in a manner known per se, be it simultaneously with driving of the compressor 4 or else separately therefrom. 

1. Method of controlling a refrigeration plant having a cooler (8) for cooling cooling air, the surface temperature (t_(K)) of the cooler (8) being measured by means of a single sensor (2) and the cooling-air temperature (t_(L)) being derived from the surface temperature (t_(K)) of the cooler via a correction factor, the refrigeration plant then being controlled on the basis of the measured temperature value (t_(K)) and the derived temperature value (t_(L)).
 2. Method according to claim 1, in which, after a predetermined tire after detection via the temperature sensor (2), according to which the cooling-air temperature incorporating the correction factor is in the desired range, the refrigeration unit is switched off or remains switched off and a fan (3) on the cooler (8) is switched on in order to lead cooling air to the cooler (8), and the actual temperature (t_(L)) of the cooling air is checked by the temperature sensor (2) in this way.
 3. Control device for a refrigeration plant having a cooler (8) for cooling cooling air, for example in a cold storage room, by means of which device the cooling-air temperature (t_(L)) is kept at a predefined value, comprising a single sensor (2) for determining the surface temperature (t_(K)) on the cooler (8) and a control unit (1) which derives the cooling-air temperature (t_(L)) from the measured surface temperature of the cooler (8) via a correction factor (K) and controls the refrigeration plant on the basis of the cooling-air temperature (t_(L)) determined in this way, in conjunction with the measured surface temperature (t_(K)) of the cooler (8).
 4. Control device according to claim 3, the temperature sensor (2) being arranged between the cooling fins (17) of the cooler (8) in the evaporator (9) of the refrigeration plant.
 5. Control device according to claims 3 and 4, the control unit (1) controlling the compressor (4), the defrost heater (6) and the fan (3) in the evaporator (9) of a refrigeration plant as a function of the measured temperature value (t_(K)) on the cooler surface.
 6. Control device according to claims 3 to 5, the control unit (1) displaying the cooling-air temperature (t_(L)), detected via the correction value, in a display as the room air temperature and, if appropriate, storing and logging the said cooling-air temperature.
 7. A method of controlling a refrigeration plant having a cooler for cooling cooling air comprising: measuring surface temperature (t_(K)) of the cooler with a single sensor; determining cooling-air temperature (t_(L)) from the surface temperature (t_(K)) of the cooler with a correction factor; and controlling operation of the cooler on the basis of the measured temperature value (t_(K)) and the cooling-air temperature value (t_(L)).
 8. The method according to claim 7, wherein after a predetermined time after detection via the temperature sensor, according to which the cooling-air temperature incorporating the correction factor is in a selected range, the refrigeration unit is switched off or remains switched off and a fan associated with the cooler is switched on to lead cooling air to the cooler, and actual temperature (t_(L)) of the cooling air is checked by the temperature sensor.
 9. A control device for a refrigeration plant having a cooler for cooling cooling air, wherein cooling-air temperature (t_(L)) is kept at a selected value comprising: a single sensor for determining the surface temperature (t_(K)) on the cooler; and a controller which determines the cooling-air temperature (t_(L)) from measured surface temperature of the cooler with a correction factor (K) and controls the refrigeration plant on the basis of the cooling-air temperature (t_(L)) in conjunction with the measured surface temperature (t_(K)) of the cooler.
 10. The control device according to claim 8, wherein the sensor is arranged between cooling fins of the cooler in an evaporator connected to the refrigeration plant.
 11. The control device according to claim 9, wherein the control unit controls a compressor, a defrost heater and a fan in an evaporator of the refrigeration plant as a function of the measured temperature value (t_(K)) on a surface of the cooler.
 12. The control device according to claim 10, wherein the control unit controls a compressor, a defrost heater and a fan in an evaporator of the refrigeration plant as a function of the measured temperature value (t_(K)) on a surface of the cooler.
 13. The control device according to claim 9, wherein the control device displays the cooling-air temperature (t_(L)), detected via the correction value, on a display as the room air temperature and, optionally, stores and logs the cooling-air temperature.
 14. The control device according to claim 10, wherein the control device displays the cooling-air temperature (t_(L)), detected via the correction value, on a display as the room air temperature and, optionally, stores and logs the cooling-air temperature.
 15. The control device according to claim 11, wherein the control device displays the cooling-air temperature (t_(L)), detected via the correction value, on a display as the room air temperature and, optionally, stores and logs the cooling-air temperature.
 16. The control device according to claim 12, wherein the control device displays the cooling-air temperature (t_(L)), detected via the correction value, on a display as the room air temperature and, optionally, stores and logs the cooling-air temperature. 